As a curable composition containing a polysiloxane including a photoacid generator, for example, Patent Document 1 discloses a study on the use of a silicone resin having an epoxy group. However, in general, a polyether structure produced by polymerization of epoxy groups is poor in heat resistance, and thus a cured product composed of a curable composition having an epoxy group has a problem in terms of heat resistance and light resistance. Consequently, in particular, development of a heat- and light-resistant resin having transparency has been strongly desired.
Furthermore, curable compositions containing a polysiloxane having a polyhedral structure containing an epoxy group have also been disclosed (Patent Documents 2 and 3). However, in these techniques, although a polysiloxane having a polyhedral structure is essentially used, many hydrocarbon units (spacers composed of an alkylene chain) are present between epoxy groups and a polysiloxane skeleton constituting a polyhedral structure, and a polyether structure produced by polymerization of epoxy groups is poor in heat resistance and light resistance. Therefore, a further improvement has been desired.
Meanwhile, for example, Patent Document 4 discloses a photocrosslinkable composition containing a compound having a hydrolyzable silyl group in its molecule. In this technique, a main chain structure thereof does not necessarily have good heat resistance or light resistance, and thus an improvement has been desired.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-238589    Patent Document 2: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2004-529984    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-359933    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2001-172514